The invention relates to a capacitive touch sensor for detecting the presence of an object within a sensing area.
The use of touch-sensitive sensors is becoming more common. Examples include the use of touch sensors in laptop computers in place of mouse pointing devices and as control panels for receiving user inputs to control a device or appliance, both domestic and portable.
Touch-sensitive sensors are frequently preferred to mechanical devices because they provide a more robust interface and are often considered to be more aesthetically pleasing. In addition, because touch-sensitive sensors require no moving parts to be accessible to a user, they are less prone to wear than their mechanical counterparts and can be provided within a sealed outer surface. This makes their use where there is a danger of dirt or fluids entering a device being controlled particularly attractive. Furthermore, unlike mechanical interfaces, touch sensitive sensors can be made transparent. There is an increasing desire to provide transparent sensors because these can be used over a display to provide a touch sensitive screen which is capable of displaying information to a user and responding to a user pointing to particular areas of the display.
A known two dimensional position sensor is described by the present inventor in WO 00/44018. This position sensor comprises an array of N by M touch keys. Each key corresponds to an intersection between a drive electrode and a sense electrode. An electrical drive signal is applied to the drive electrode. The degree of capacitive coupling of this drive signal to the sense electrode is determined by measuring the amount of charge transferred to the sense electrode in response to changes in the drive signal. The degree of capacitive coupling between the electrodes at a given key is dependant on the presence of objects in the vicinity of that key since these will modify the electric field pattern between the electrodes. Some objects, e.g. conductive water films, will increase the capacitive coupling. Other objects, e.g. a human having a significant capacitive coupling to ground, will decrease the capacitive coupling. This is because charge can be sunk to ground through the adjacent object, rather than through the sense electrode.
The array of keys described in WO 00/44018 is a matrixed array. This means a single drive electrode is associated with the keys in a given column and a single sense electrode is associated with the keys in a given row. This reduces the number of drive and sense channels required since a single drive channel simultaneously drives all of the keys in a given column and a single sense channel senses all of the keys in a given row. The capacitive coupling between the electrodes at the positions of the different keys can be determined by driving the appropriate column and sensing the appropriate row. For example, to determine the capacitive coupling between the electrodes associated with a key at the intersection of column 2 and row 3, the drive signal is applied to the drive electrode of column 2 while the sense channel associated with the sense electrode of row 3 is active. The output from the active sense channel reflects the capacitive coupling between the electrodes associated with the key under investigation. It does not matter that the drive signal is applied to other keys in column 2 because the rows associated with these keys are not being sensed. Similarly, it does not matter that the other keys in row 3 are being sensed because these keys are not being driven. In this way the different keys can be scanned by sequencing through different combinations of drive and sense channels.
The two dimensional position sensor described in WO 00/44018 is a robust and efficient device. However, an important feature of this type of position sensor which contributes to its good performance is the fact that during the transfer of charge from the driven electrode to the sensing electrode, the sensing electrode should appear as a virtual ground. Each sense channel includes a charge detector for determining the amount of charge transferred in response to the changing drive signal (i.e. the degree of AC capacitive coupling). It is important that the sense electrode presents a low impedance node because this allow charge to be efficiently sunk to the charge detector. If the sense electrode does not present a low input impedance, charge flows induced in the sense channel by the changing drive signal appear as voltage pulses on the sense electrode. This makes the sensor susceptible to walk-by interference from objects near to the wiring connecting between the sense electrode and the charge detector. This is because an object near the wiring can absorb some of the signal from the wiring connection and so reduce the signal supplied to the charge detector. This can appear to the control circuitry as a reduction in capacitive coupling at the key (i.e. a detection event), even though the object is adjacent the wiring rather than near the sense electrode. Thus a hand reaching across the wiring of one row of electrodes to activate a key in another row can lead to an erroneous output. Furthermore, if the sense channel has significant input impedance the length of wiring used in the sensor becomes a factor in determining the gain of the circuit. This is because the wiring will cause some of the sense signal to “bleed off” capacitively into free space, adjacent wires, and ground, thus forming a capacitance divider circuit together with the capacitance coupling between the key.
The low impedance characteristic of the charge detection circuitry used in WO 00/44018 means that the above identified problems are reduced. However, a problem arises in the context of transparent touch sensors since most transparent conductors of electricity that can be used to form the drive and sense electrodes in a position sensor of the kind described in WO 00/44018 (e.g., Indium Tin Oxide (ITO)) are highly resistive compared to copper wiring. For example, 300 ohms per square is customary in optimally transparent ITO films. This means unlike materials such as copper, transparent conductors cannot generally be formed into electrodes and associated wiring having negligible resistance (at least for thicknesses at which they remain substantially transparent). Thus even if the charge detection circuitry itself presents a suitably low impedance, the material comprising the columns of drive electrodes and the rows of sense electrodes themselves (and any associated wiring traces made of the same material) may not. Thus devices of the type described in WO 00/44018 in which the electrodes are to be formed by a resistive conductor (e.g. transparent conductors) are difficult to implement reliably.
Furthermore, of considerable importance in many touch panel designs is the parallel ability to sense capacitive touch keys adjacent a touch screen area. These kinds of panel keys are substantially described in WO 00/44018 in conjunction with touch buttons on a dielectric surface, using copper or other metallic, low resistance wiring. The desire to incorporate both low resistance wiring with high resistance clear-conductor wiring can create circuit design problems, usually necessitating the use of two different types or performances of capacitive circuits.